I have the same issue it seems. I was playing straight into the amp (without HD500) and as I was turning up the volume I got a loss in power and a bad hum. I reset the amp and turned it back on, same issue but noticed a pop when i took it out of standby. Now the amp has very little power and is very dirty. Even voicing 1 is really dirty even with the drive at 1 in full power mode. Class A seems to have a louder hum. I had the tubes changed and the amp biased and same results. I even re-installed the firmware. I love this amp and the fact that it is a paper weight breaks my heart. I have it narrowed down to the processing board or the transformer. Any help would be much appreciated.